Refractory mold material



Patented Aug, 20, 1940 PATENT o l-"ica r narimo'ronr MOLD MATERIALAlbert w. Merrick, New York, my and Paul H.

- Beier, Chicago,

Hit, .assignors to 'Austenal Laboratories, Inc, New York, N. Y., acorporation of New York No Drawing. App ca ion November 19, 1937,-Serial N0. 175,428

I I 3 Citizens.

The present invention relates to an improved refractory mold material,and more particularly to an improved refractory mold material for incasting high temperature fusing metals and alloys. I

The particular improved refractory mold matei which we shall describehereinafter is particularly useful for the production of dental molds inwhich dentures and similar appliances and dental elements are adapted tobe cast, but it is to be understood that the invention may be em- Iployed for molding other articles of high teiriper ature fusing metalsand alloys, such as articles of jewelry, and for molding other materialsas suitable anddesired.

The metals and metal alloysv used for dental and other. work shrink onpassing from the casting temperature, at which they are, of course, inmolten condition, to ordinary room tempera hire; and this shrinkage mustbe compensated for For the production of molds and the like for castinglow temperature fusing metals and alioys, mold materials containingrelatively high percentages of plaster of Paris have been usedextensively. However, due to their low resistivity against heat, thesemold materials cannot be used for the higher temperature fusing metalsand alloys. They will break down or disintegrate at temperatures aboveabout 1200 F.

It previously has been proposed to secure the desired expansion tocompensate for theoesting shrinkage which takes place as above set iorthby means of investment compositions comprising preformed trldymite orcristobalite with a relatively large percentage of a binder consistingof plaster of Paris or the like.

With investment compositions of the character last mentioned, maximumexpansion is secured at temperature too low to make such materialssuitable for use with high temperature fusing metals and metal alloys.For example, if straight preformed cristobalite is used with arelatively large percentage of plaster of Paris, maximum expanslon issecured at about 1200 F. This may be suitable for low temperature fusingmetals and alloys, such as gold, which fuses at about 1950 F., but suchinvestment materials are not suited for use with high temperature fusingmetals or alloys such, for example, asthe alloys disclosed .in'

' Charles H. Prange Patent bid-1,958,446, patented May 15, 1934, and inthe copending application of Charles H. Prange, Serial No. 725,651,filed May 14, 1934. These latter alloys are known on (owe-1 s) themarket as.Vitallium, and have fusing temperatures of about 2500 F.

We have found it difficult to secure complete castings of hightemperature fusing metals and alloys such as those above set forth,particularly in the case of larger and thinner pieces, when the moldtemperature is too low; the reason appear ing to be that the relativelylarge temperature difierence between the metal or alloy and the in=vestment or mold materiel causes the metal or alloy to chill prematurelybefore filling the mold.

The introduction in molten condition. of high temperature fusing metalsor alloys of the sort herein referred to into molds formed of an investment composition comprising preformed tridymite or preformedcristobalite and a, binder of plaster of Paris or the like in relativelylarge percentage results in breaking down or disintegration of theplaster of Paris binder, extensive porosity, and lack of solidity in theresulting cast= lugs, due to the high casting temperatures of these hightemperature fusing metals and alloys.

Due to the thermal expansion to which the silica of refractory moldmaterials consisting principally of ground quartz is subjected duringheating and while changing from alpha-quartz to beta-quartz, andespecially during the .change of beta-quartz into tridymite, the desiredexpansion of the mold necessary to compensate for easting shrinkage orcontraction of the metal to be poured into the mold is obtained, butthis trans formation takes place only at very high temperatures of about2300 F., and the reaction'or con version process progresses very slowlyor lazily.

Where temperatures as high as this are necessary to bring about thedesired expansion, it will be noted that they approach closely themelting point of the high fusingmetals and alloys. Such hightemperatures result in coarse crystallization and a tendency towardsdendritic effects and extensive porosity in the castings, which ishighly undesirable.

It is one of'the primary objects of the present invention to provide animproved refractary mold material in which the desired expansion of themold material to compensate for casting shrinkage is brought about atessentially lower temperatures than 2300 F., and which mold'material hashigh reslstivity'against heat and thereby is admirably adapted for usein casting the high temhereinbefore mentioned.

Another object; of the present invention is to provide an improvedrefractory mold material having the advantageous characteristics aboveset perature fusing metals and alloys of the class forth, and which hasthe further advantage of avoiding interference with the setting actionof plaster of Paris or organic silicate binders or other binders .orsetting agents in the mold material.

Another object of the present invention is to provide an improvedrefractory mold material in which the inversion of the quartz, whichtakes place in increasing amounts as the temperature of the'mold rises,and the resulting expansion to compensate for casting shrinkage arelimited by the final temperature reached and are not further increasedbyprolonged heating at this temperature. In other words, the desiredexpansion is secured at the final temperature for securing thatexpansion and is independent of time after the final temperature for thedesired expansion is reached. This is highly advantageous in that itenables the operator to bring a large number of molds up to thetemperature for securing the desired expansion, and then to introducethe molten metal to be cast into the molds, one after another, withouthaving different expansions in the various molds due to thetime-interval after the final temperature is reached; for example,between the filling of the first molds and the filling of the lastmolds. I

Another object is to provide an improved refractory mold material inwhich the conversion and expansion herein referred to are very rapid,particularly under the conditions that we are using it; that is, theconversion takes place comunderstood that modifications and variationsfrom the specific embodiments selected for illustration are contemplatedwithin the scope of the appended claims.

One satisfactory formula embodying the present invention is as follows:

- Parts "Hydrocal" Ground quar 12 Flint 3 Magnesium sulphate (groundfine) A; Glass (ground fine) The ground quartz, which constitutes therefractory aggregate and makes up the principal portion of the moldmaterial, is preferably ground fine, i. e. to about mesh or finer.

The Hydrocal, which is a plaster base mate- .rial well known on themarket, constitutes a binding agent for the mold material. Thisparticular material has 'an alpha plaster base, and is resistant tomoderately high temperatures without breaking down. It is to "beunderstood, however, that any other grade of plaster or plaster basematerial may be used within the scope of the present invention.

The glass content is preferably an ordinary soda lime glass, and servesto introduce sodium oxide in insoluble form. It is preferably groundfine, i. e., to about 200 mesh or finer, to secure a better mix andbetter uniformity of distribution of the same throughout thecomposition. The sodium oxide which is introduced by this materialserves to bring about, by conversion of the silica of the refractorymold materiaL con- 'sisting principally of ground quartz, into indymite, the desired expansion of the mold material at essentially lowertemperatures than 2300 F. The insoluble form in which the sodium oxideis introduced by employing soda lime glass as set forth is advantageousin that objectionable interference with the setting action of theplaster setting agent is avoided. In other words, the sodium oxide ininsoluble form does not objectionably hasten the setting action of themold-material, whereas an agent such as water glass (sodium silicate)which introduces the sodium oxide in soluble form will bring about, byconversion of the silica, the desired expansion at essentially lowertemperatures than 2300 F., but does greatly accelerate the settingaction of the binding agent.

The magnesium sulphate is illustrative of one suitable accelerator foraccelerating the setting action. Sodium chloride, magnesium oxide, heat,or any other accelerating agent well known in the art, may be employedwithin the scope of the present invention.

Due to the small amount of magnesium sulphate and also the small amountof glass used,

it is preferable to make pre-mixes of the magnesium sulphate and theglass with ground quartz in making up the mold material.

A pre-mix of glass, hereinafter called A, may

beformed as follows:

. i I A I L I Parts Glass (powdered) 1 Ground quartz (preferably aboutmesh)..- 1

A pre-mix of magnesium sulphate, hereinafter called B, may be formed asfollows:

Parts Magnesium sulphate (ground fine) 1 Ground quartz (preferably about100 mesh) 2 With the pre-mixes of glass and magnesium sulphate, theformula first above set out then We prefer to add to the above formulaY4 part sugar or other suitable temporary binder, but this may varywidely within the scope of the present invention.

In making up the mold material, the ingredients are thoroughly mixed inthe mill and subsequently water is added to the composition in an amountto bring the same to the desired consistency. After the composition isbrought to the desired consistency, it is molded or shaped into whateverform is desired, and in molding or shaping the material it is preferablycompacted by vibration to bring the particles into contact and toproduce a hard and relatively non-shrinking mass. The compacting of themixture by vibration may be accomplished as by means of a suitablerevolving cam which will raise the mixture slowly and drop it abruptly,thereby compacting the particles into contact without agitating themixture. The particular manner of compacting the mixture to obtain thedesired results may be accomplished in any other suitable or preferredmanner.

Other percentages of ordinary soda lime glass than the particularpercentage given may be employed with the plaster of Paris investmentabove set forth. We contemplate particularly ordinary soda lime glassfrom about 1% to about 5% with this plaster of Paris investment, butvariations outside of that range are contemplated as suitable anddesired within the scope of the broader aspects of the invention.

Another suitable refractory mold material em.- bodying the presentinvention may be made by using substantially 100 parts, or, more particularly, about 99.9 parts ground quartz and .1 part magnesium oxide orother organic or inorganic accelerator with an organic binder, such astetraethyl silicate in amount to form a heavy paste, mixed with fromabout to about 3% ordinary soda lime glass.

The soda lime glass which we employ is obtained from the PittsburghPlate Glass Company, and introduces a definite percentage of sodiumoxide for a definite percentage of such glass employed. Other sodiumlime glasses may introduce different percentages of sodium oxide for adefinite percentage of such glass employed, and therefore we contemplatethe full range of variations of the glass content of the material tomeet these variations.

The glass content is ground fine, as set forth in connection with thefirst described embodiment of the invention, and we find that with 2 ofsoda lime glass of the character above set forth, the desired expansionto compensate for shrinkage of the metal in casting the same in the moldis secured at about 1800 F. With a soda lime glass content of thedesired expansion to compensate for shrinkage of the metal and to assurethe desired accuracy of fit is secured at about 2200 F. With soda limeglass,

of the character above set forth, present in amount up to" about 3%, theinversion process or transformation from alpha-quartz to betaquartz, andfrom betaquartz to tridymite,'is a function of temperature andindependent of the elapse of further time after the required degree ofexpansion is completed, with the resulting advantages previously setforth. With a soda lime glass content above about 3%, the inversion ortransformation process is a function not only of temperature, but alsois a function of the elapse of further time after the required degree ofexpansion is completed. We prefer to limit the amount of glass employedto a percentage which will give the desired expansion as a function oftemperature and independent of the elapse of further time after therequired degree of expansion is completed, because, for example, of theimpossibility of casting in all of a plurality of heated molds at thesame time.

In making up the mold or mold part of the latter composition, thetetra-ethyl silicate binder is added in an amount to bring thecomposition to the desired consistency, as explained in connection withthe first embodiment of the invention. As before, after the compositionis brought to the desired consistency, it is molded or shaped into thedesired form, and in molding or shaping the material it is compacted byvibration to bring the particles into contact and to produce a hard andrelatively non-shrinking mass. The compacting or vibration of thematerial settles out or removes any excess amount of the tetraethylsilicate binder. As befor, the compacting of the mixture by. vibrationmay be accomplished as by means of a. suitable revolving cam which willraise the mixture slowly and drop it abruptly, thereby compacting theparticles into contact without agitating the mixture.

Another suitable refractory mold material embodying the presentinvention may be made of substantially 100 parts ground quartz, or, moreparticularly, about 99.9 parts ground quartz with .1 part accelerator,or by using heat as the accelerator as herein described, with silicateof soda or water glass as a binder. This binder is in the form of asoluble binder, with the disadvantages previously set forth, but iscontemplated within the scope of the broader aspects of our invention.The accelerator, other than heat, as above referred to, may be calciumoxidethat is, lime or compounds containing it, such as Portland cement,which are known to act as acceleratorsand various other substances, allof which tend to bring about a setting action with silicate of soda.

With this latter composition, the. silicate of soda is employed. as abinder and serves the additional purpose of introducing sodium oxide thedesired expansion of the mold material to compensate for castingshrinkage at essentially lower temperatures than 2300 F. While thislatter composition requires more time in the setting process, itprovides the desired expansion at lower temperatures, and, in thatrespect, embodies the broader aspects of the present invention.

In making up the mold or mold part of the last mentioned composition,the silicate of soda or water glass binder is added in amount to bringthe composition to thedesired consistency, whereupon the composition ismolded or shaped to the desired form and preferably compacted byvibration as set forth in connection with the preceding embodiments ofthe invention.

The expansion which we provide for securing at essentially lowertemperatures than 2300 F. compensates fully and accurately for shrinkageof the high temperature fusing metals and alloys herein mentioned incooling from the casting temperature to ordinary room temperature. Themolten metal is introduced into the molds formed of our improvedrefractory mold material, which said molds are heated to the temperatureto provide the desired expansion.

Upon heating quartz in the presence of the powdered glass, after thealpha-beta -transformation has occurred, some quartz is inverted totridymite, depending upon the amount of glass, or other agent, employed.These changes all result in expansion, and, on cooling, the materialcontracts to its original dimensions, except for the quartz-tridymiteinversion, which change is irreversible. Therefore, in practice, it isdesirable to use such proportions of the controlling agent, i. e.,powdered glass or other controlling agent as herein described, as willproduce the desired net results, which is accomplished by varying theamount of the agent used in various combinations of mold materials, andto suit or compensate for the contraction or shrinkage of the variousalloys or other materials cast.

Instead of using soda lime glass, silicate of soda, or water glass forbringing about, by conversion of the silica, consisting principally ofground quartz, the desired expansion of the mold material at essentiallylower temperatures than 2300 F., other ingredients are contemplated forthis purpose within the broader aspects of our present invention. Forexample, the

'to bring about, by the conversion of. the silica,

oxides and hydroxides of the alkali metals which include sodium,potassium, lithium, caesium and rubidium, may be used .to bring about,by conversion of the silica, consisting principally of ground quartz,the desired expansion at essentially lower temperatures than 2300 F. Thelast two named alkali metals are unimportant from a practical point. ofview, being rare elements. The other three are entirely practicable touse as they all forrn silicates and these silicates are availablecommercially. We, therefore, contemplate particularly the use ofmaterials containing oxides of the .alkali. metals, especially sodium,potassium and lithium, as well as insoluble glasses and other compoundscontaining the oxides of these materials.

Where potassium oxide is used to bring about the desired expansion atessentially lower temperatures, it may be introduced by using groundpotash glass instead of soda lime glass as heretofore described.

As already set forth, we secure the conversion and desired expansion inour improved refractory mold material very rapidly, particularly underthe conditions that we are using it-that is, the conversion takes .placecompletely to the extent that we have added a definite proportion ofglass within the time consumed in bringing the mold up to the castingtemperature. To the best of our knowledge, the inversion of quartz intotridymite is a gradual process which takes place progressively, and itprobably begins to some extent within a relatively short time but takesa very long time, ordinarily about seventy-two hours or more to bringabout complete conversion of all of the quartz into tridymite, whereaswe may secure complete conversion for the amount of ground glass orother agent which is incorporated and for the temperature to which theinvestment is heated, and thereby the desired expansion, in about twoand one-half hours or less. This particular time is referred to forpurposes of illustrating the advantage in this respect, and, of course,is not in anywise intended to limit the present invention.

We do not intend to be limited to the precise ingredients, nor do weintend to be limited to the precise proportions herein set forth.

We claim:

1. Refractory mold material for making molds into which high meltingpoint metals and alloys are to be cast, comprised essentially of groundquartz and comprising, in addition thereto, a binding agent and up toapproximately 1.0 percent of an alkali oxide selected from the groupconsisting of sodium, potassium and lithium introduced as a constituentof an inert carrier comprising soda lime glass, said inert carrier beingsubstantially insoluble and thereby avoiding any disturbance of thenormal setting of the binding agent, said alkali oxide accelerating thetransformation of quartz into tridymite at reduced temperature and beingintroduced in such amount that the resulting expansion is substantiallylimited at the final casting temperature and is not further increased bycontinued heating at the casting temperature for an interval of timeafter the casting temperature is reached.

2. Refractory mold material for making molds into which high meltingpoint metals and alloys are to be cast, comprised essentially of groundquartz and comprising, in addition thereto. a binding agent selectedfrom the group consisting of tetra-ethyl silicate and plaster of Parisand up to approximately 1.0 percent of an alkali oxide selected from thegroup consisting of sodium, potassium and lithium introduced" as a constituent of an inert carrier comprising soda lime glass, said inertcarrier being substantially insoluble and thereby avoiding anydisturbance of the normal setting of the binding agent, said alkalioxide accelerating the transformation of quartz into tridymite atreduced temperature and being introduced in such amount that theresulting expansion is substantially limited at the final castingtemperature and is not further increased by continued heating at thecasting temperature for'an interval of time after the castingtemperature is reached.

3. Refractory mold material for making molds into which high meltingpoint metals and alloys are to be cast, comprised essentially of groundquartz and comprising, in addition thereto, a binding agent selectedfrom the group consisting of tetra-ethyl silicate and plaster of Parisand up to approximately 1.0 percent of an alkali oxide selected from thegroup consisting of sodium, potassium and lithium introduced as aconstituent of an inert carrier comprising soda lime glass, said inertcarrier being substantially insoluble and thereby avoiding anydisturbance of the normal setting of the binding agent, said alkalioxide accelerating the transformation of quartz into tridymite atreduced temperature and being introduced in such amount that conversionof the quartz into tridymite takes place to the extent necessary toproduce the desired expansion.

ALBERT W. MERRICK. PAUL H. BEIER.

